xii 



INTRODUCTION. 



which is a membranous, and not elastic tube, 

 the sides of which are marked with transverse 

 lines or spiral dots. These vary in form more 



WOODY FIBRE. SPIRAL VESSEL. DUCT. 



than either of the last two, of one or other of 

 which they appear, in many instances, to be 

 modifications. The use of vessels generally ap- 

 pears to be to convey the ascending and descend- 

 ing currents of sap and air ; but, owing to their 

 extreme minuteness, and the difficulty of observa- 

 tion, there is much difference of opinion as to the 

 exact purposes which they severally answer in the 

 economy of plants. 



Trees are either endogenous (growing inwardly), 

 or exogenous (growing outwardly). All the trees 

 described in these volumes belong to the latter 

 class. A transverse section of the trunk of any 

 one of these trees will present the appearance 

 about to be described, varying more or less ac- 



